The present invention relates to a coding unit and method of coding an audio signal by using orthogonal transform by using an adaptive transformation such as a discrete cosine tansformation (DCT), a discrete Fourier transformation (DFT) and the like, and more particularly to a coding unit and method in which a minimum audible acoustic level is set to a predetermined value adaptive to a power level for reducing a coding rate.
When an audio signal is compressed and coded, a conventional system excepts "an unnecessary component" of the signal component as being inaudible for effectively reducing a code rate.
A person having normal hearing ability can generally hear sounds within the area enclosed by two lines representing the maximum and minimum audible levels, respectively. This audible area is illustrated in FIG. 1 as an area with hatching. Accordingly, signals representing sounds under the minimum audible level can be ignored.
A signal on the time axis is transformed on a frequency axis by the orthogonal transformation and this resultant coefficient may distribute from a full scale value (if 16 bit data, it is 2.sup.15) to an operational accuracy value. In the conventional method, the full scale value is set to the maximum audible level (120 dB Sound Pressure Level --S.P.L.--) and the minimum audible level (about 0 dB S.P.L. at 1 kHz) is determined according to the maximum level, as shown in FIG. 1, and samples (coefficient) under the minimum audible level are omitted. However, it is very rare for actual music signals to hit an upper value of the full scale.
Accordingly, when a signal source having an average of a signal level at about 60 dB which is calculated on the time axis or the frequency axis, is set to 80 dB S.P.L. of an average at reproduction, as shown in FIG. 2(a), an oblique lined portion omitted in coding as an inaudible sample, is as shown in FIG. 2(b) at an actual reproduction, so that a part of the actual audible portion is lost. Namely, an average power dB that is calculated in a time range, can be used in a frequency range as it is.
In this case, the minimum audible level (hereinafter, the level is called a threshold level) needs to be set to be lowered by 20 dB. In contrast, when the signal of 80 dB of an average level is reproduced by 80 dB S.P.L. under the threshold level, is set before, as shown in FIGS. 3(a) and 3(b), there remains signals which cannot be actually audible.
As described above, a conventional method has problems that too many signals are lost when the level of reproduction is set higher, and in contrast, unnecessary signals are left when the level of reproduction is set lower.